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Discussion Starter · #1 ·
First off, hello to all the forum members. I've visited the forum a few times through google searches when looking for torque specs, etc but this time I need a little more guidance. Hopefully as a member I can contribute from my experience now as well.

My 2004 5.3 silverado has about 170k on it and has slowly been developing an intermittent misfire at idle. When I bought the truck second hand it would miss a couple times a minute but in the last few months it has become much more prominent. It is getting to the point where it can be felt every few seconds in the steering wheel. I'd like to fix it before it fouls plugs, clogs cats, etc without just throwing time and money at it until it goes away. I will be trying to find a vacuum leak with carb cleaner this evening. If that doesn't lead to anything I will look into the coil packs and o2 sensors. Thanks for your help and input!

*Misfire conditions/truck conditions*
-Only noticible at idle
-No hesitation with acceleration
-No codes ( I kind of wish I would get one so I'd have some direction )
-Plugs and wires were replaced within 5k miles
-Recently cleaned and checked the operation of the injectors
-Air filter replaced within the year
-MAF sensor gets cleaned every few months
-No odors, smoke, etc from exhaust
-Doesn't appear to be relative weather/temp
 

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Discussion Starter · #4 ·
I picked up a couple can's of carb cleaner today and I have tomorrow off. Hopefully I won't be left scratching my head so I can post some definitive results.
 

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Discussion Starter · #5 ·
Two cans of carb cleaner later and I am at the same place I started. It doesn't mean there isn't a leak, it just means there isn't one that I can reasonably find with the cleaner. I'll re torque everything and see if that makes any difference in the idle.
 

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How did you verify the operation of the injectors?
 

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Discussion Starter · #7 ·
Silvery13 said:
How did you verify the operation of the injectors?
I pulsed pressurized carb cleaner through them individually.

I was going to clean the IAC valve yesterday but 04 models the IAC and TPS riveted to the throttle body. If either goes out the whole unit needs replaced rather than just the bad sensor. There was some buildup of dirt that mostly led in to the IAC port on the front side of the TB so I am going to find a way to clean it. My initial plan is to drill out the rivets to get access to the IAC valve and replace them with 4 small bolts, nut, and washer combos. I haven't seen that anyone has done it yet but I'll continue searching.
 

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Discussion Starter · #8 ·
It turns out that they removed the IAC on the 5.3s when they went to the "drive by wire" throttle body. I still took the TB off and cleaned it up though. None of the coil packs responded to being sprayed with water either. Next fill up I am going to see if it running lower octane makes any difference. The head scratching continues.
 

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I have exactly the same issue. A lot of others do too, and it's driving me nuts there isn't a well known solution for this.

Did you do the intake gasket replacement? If not, was it because no leaks were detected in the carb cleaner test?

Does anyone know if an intake gasket leak results in idle RPM fluctuation on the tac or just a roug feeling idle?

I'm seriously considering pulling my injectors and having them professionally tested/rebuilt by a local shop that specializes ONLY in injectors.

Id rather the gasket be the fix since that's $30 vs the $180 for the injectors..

I really hate these fishing trips.
 

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Discussion Starter · #11 ·
gsauer80 said:
I have exactly the same issue. A lot of others do too, and it's driving me nuts there isn't a well known solution for this.

Did you do the intake gasket replacement? If not, was it because no leaks were detected in the carb cleaner test?

Does anyone know if an intake gasket leak results in idle RPM fluctuation on the tac or just a roug feeling idle?

I'm seriously considering pulling my injectors and having them professionally tested/rebuilt by a local shop that specializes ONLY in injectors.

Id rather the gasket be the fix since that's $30 vs the $180 for the injectors..

I really hate these fishing trips.
I haven't replaced the intake gaskets yet since I didn't find anything with the vac leak tests. The truck still runs fine otherwise so I'll keep chipping away at it until I find something. This weekend 'Ill probably physically check the coil packs one at a time with the engine running.

needmods02 said:
Weird question....
What plugs and did you check your gaps?
Well, I guess that was 2.
I'm running NGK Iridium IX plus and I did re gap them out of the box. The idle issue was there before the plug change and there was was no difference afterwards.
 

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Discussion Starter · #12 ·
Today I checked the coils with an inline spark tester and discovered that spark is strong to all eight plugs and my left thumb. The idle stumble was unchanged even with the added resistance from the gap of the tester. I'll be checking fuel pressure and the fuel filter next. Updates to come.
 

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Just a heads up, if you can't find a fuel filter you might not have a replacable one for your truck. I have an 05 and the filter is built into the fuel pump. You have to change the whole pump. Make sure you find that out before you order one online.

I'm having misfires and rough idle also so keep posting your progress plz. Thanks
 

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Discussion Starter · #14 ·
Thanks for the heads up. I just read that the 04's aren't replaceable either. Supposedly it was implemented to keep up with emisions constraints. My truck was built in Canada so maybe I'll get lucky and have a replaceable one.
 

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Discussion Starter · #15 ·
The fuel pressure checked out good at 53-56 psi. Checking the ECTsenseor(s) operation and then probably change the oxygen sensors. I'm sure they are well past their expected life anyway. I did run it until the gas light came on and then filled the tank with 3/4 ethanol free gas to see how it runs. More updates on the way..
 

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Discussion Starter · #17 ·
Still nothing. I did another vac leak test with carb cleaner and it had a code the next time I drove it. Unfortunately the code dropped before I was able to get it read. Lately it does seem to idle more roughly in the high idle time after the engine initially cranks. Either the issue is getting worse or the cool weather is affecting it. Maybe it is a vac leak on the intake that closes up as the engine warms the gaskets up. That seems to be fairly common.
 

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Discussion Starter · #19 ·
Last week I tried another vac test with the carb cleaner. This time I was much more generous with the amount that I sprayed. I never heard the idle change but when I got in th truck I noticed a code was set. Unfortunately it was gone by the the time I had gone to work but I went off the hunch that there was a vac leak somewhere. I pulled the intake and replaced the intake manifold gasket on sunday. While I had access I cleaned the knock sensors, applied dielectric grease to the connections, and sealed the covers in place with rtv sealant. After getting everything back together the idle seemed to be much smoother but I need to put some miles on it and see where it is at.
 

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Discussion Starter · #20 ·
After taking a few trips and getting the engine up to operating temps I can say that nothing changed. The random misfire is still there. At least I got the knock sensors cleaned up and sealed and I eliminated intake as being the culprit. Next up will be the oxygen sensors since they are all original.
 
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